Recently, electric welding has been widely used for the seaming of tinplate can bodies in the field of food cans, aerosol cans, and miscellaneous cans, instead of soldering. In the seaming of the tinplate can body, it is desirable to decrease the tin coating weight in the tinplate because tin used for electrotinplate is very expensive and there is concern over the exhaustion of tin resources. However, the weldability of the tinplate becomes gradually poor with a decrease of the tin coating weight.
From the background described above, the development of a welded can material which is cheaper than conventional electrotinplate, is easily welded at high speed without the removal of the plated layer and is excellent in corrosion resistance after lacquering, has been required in the field of food cans. Over the past few years, various surface treated steel sheets have been proposed for welded can materials which can be easily welded at high speed without the removal of the plated layer and is cheaper than tinplate. For instance, the following surface treated steel sheets have been proposed: (a) Lightly tin plated steel sheet (LTS) with below about 1.0 g/m.sup.2 of tin which is reflowed or unreflowed after tinplating (Japanese Patent Publication Nos. Sho 56-3440, Sho 56-54070, Sho 57-55800, and Laid-Open Japanese Patent Application Nos. Sho 56-75589, Sho 56-130487, Sho 56-156788, Sho 57-101694, Sho 57-185997, Sho 57-192294, Sho 57-192295, and Sho 55-69297). (b) Nickel preplated LTS with below about 1.0 g/m.sup.2 of tin (Laid-Open Japanese Patent Application Nos. Sho 57-23091, Sho 57-67196, Sho 57-110685, Sho 57-177991, Sho 57-200592, Sho 57-203797, Sho 60-33362, and Sho 60-56074). (c) Nickel plated steel sheet with chromate film or phosphate film (Laid-Open Japanese Patent Application Nos. 56-116885, Sho 56-169788, Sho 57-2892, Sho 57-2895, Sho 57-2896, Sho 57-2897, Sho 57-35697, Sho 57-35698).
However, LTS and nickel preplated LTS above identified as (a) and (b) have a narrower current range for sound welding than that in tinplate, although these can be welded without the removal of the plated layer. The reason why the current range for sound welding in LTS and nickel preplated LTS is narrower than in tinplate is considered to be that the amount of free metallic tin in these is smaller than that in tinplate and also further decreases because of the change of plated free metallic tin to iron-tin alloy or iron-tin-nickel alloy by heating for lacquer curing or reflowing after tinplating.
An increase of the tin coating weight in LTS and nickel plated LTS is contrary to the development of cheaper welded can material than tinplate, although the weldability and the corrosion resistance after lacquering are improved with an increase of it. An increase in the amount of nickel in nickel preplated LTS improves the corrosion resistance after lacquering, but does not improve the weldability, because the amount of free metallic tin decreases by the formation of tin-nickel alloy during aging at room temperature or by the formation of iron tin-nickel alloy during reflowing nickel preplated LTS.
Nickel plated steel sheet with chromate film or phosphate film identified above as (c) also has a narrower current range for sound welding than that in tinplate, LTS, or nickel preplated LTS.
Furthermore, the corrosion resistance of nickel plated steel sheet is poor, although the lacquer adhesion is good. Particularly, pitting corrosion in the defective part of the lacquered nickel plate steel sheet may occur easily in acidic foods such as tomato juice because the electric potential of nickel is more noble than that of steel sheet.
As described above, the various surface treated steel sheets proposed in (a), (b), and (c) have various problems in production cost and characteristics as a welded can material which can be easily welded without the removal of the plated layer at high speed.
Accordingly, it is the first objective of the present invention to provide a thin tin and nickel plated steel sheet having excellent corrosion resistance after lacquering and excellent weldability.
It is the second objective of the present invention to provide a method for the continuous production of a thin tin and nickel plated steel sheet having excellent characteristics as described above.